The 139.5 GHz EPR Spectrum of p2l ras complexed with Mn(H) and guanosine 5'(O,yimido)-triphosphate, an analog of GTP, has been obtained. p2l was selectively labeled with 3[1701threonine to a final enrichment of 30%. The Mn(II)-170 hyperfine interaction is measured as 0.11 q 0.04 mT, the smallest Mn(II)- 170 hyperfine value yet reported. Ab initio calculations indicate that this value is consistent with direct coordination of the hydroxyl group of Thr35 and provide an estimate for the Mn(H)-170 bond length of 2.7 A. The hyperfine coupling constant and bond length reported here contrast with more typical values of relevant Mn(II)-170 coordination complexes (-0.25 mT and -2.2 A, respectively). These results suggest that the nucleotide-induced codformational. change, which is known to encompass the region of p2l involving Thr35, is not driven by coordination of the hydroxyl group.